In the manufacture of tissue products, such as facial tissue, bath tissue, paper towels, dinner napkins and the like, a wide variety of product properties are imparted to the final product through the use of chemical additives. One common attribute imparted to tissue sheets through the use of chemical additives is softness. There are two types of softness that are typically imparted to tissue sheets through the use of chemical additives. The two types are bulk softness and topical or surface softness.
Bulk softness may be achieved by a chemical debonding agent. Such debonding agents are typically quaternary ammonium entities containing long chain alkyl groups. The cationic quaternary ammonium entity allows for the agent to be retained on the cellulose via ionic bonding to anionic groups on the cellulose fibers. The long chain alkyl groups provide softness to the tissue sheet by disrupting fiber-to-fiber hydrogen bonds within the tissue sheet.
Such disruption of fiber-to-fiber bonds provides a two-fold purpose in increasing the softness of the tissue sheet. First, the reduction in hydrogen bonding produces a reduction in tensile strength thereby reducing the stiffness of the tissue sheet. Secondly, the debonded fibers provide a surface nap to the tissue sheet enhancing the “fuzziness” of the tissue sheet. This tissue sheet fuzziness may also be created through use of creping as well, where sufficient interfiber bonds are broken at the outer tissue surface to provide a plethora of free fiber ends on the tissue surface.
Most bulk softener and debonder agents are added in the wet end of the tissue making process. The agents are typically added prior to the formation of the tissue sheet while the pulp fibers are in a slurry of water, typically at a consistency of about 5% or less. A specific limitation of wet end chemical additive addition may be a need for the chemical additives to possess a charge, cationic, anionic or amphiphilic. The cationic charge of the chemical additive is attracted to the anionic charge of the pulp fibers, allowing for the chemical additives to be retained on the pulp fibers. Where anionic chemical additives are used, a cationic promoter may be required to retain the chemical additives on the pulp fibers. A host of additional chemical additives may also be added in the wet end of the tissue making process to help modify the tissue product properties including, but not limited to, wet strength agents, dry strength agents, sizing agents, opacifiers, and the like.
A multi-layered tissue structure may be utilized to enhance the softness of the tissue sheet. In one embodiment of the present invention, a thin layer of strong softwood kraft pulp fibers is used in the inner layer to provide the necessary tensile strength for the tissue product. The outer layers of such structures may be composed of shorter hardwood kraft pulp fibers while the inner layer or layers may be composed of longer softwood kraft pulp fibers. The hardwood kraft pulp fibers may be treated with a debonding agent and the softwood kraft pulp fibers may be treated with a strength agent. Such chemical additive additions may be accomplished in the wet end of the tissue making process by adding the chemical additives to the individual pulp fiber slurries. This may be accomplished as well with blended pulp fiber furnishes as described in U.S. Pat. No. 5,785,813, issued on Jul. 28, 1998 to Smith et al.
One limitation associated with wet end chemical additive addition is the limited availability of adequate bonding sites on the pulp fibers to which the chemical additives may attach themselves. Under such circumstances, the various molecules of the wet end chemical additive or additives compete for the limited available bonding sites, resulting in incomplete retention of the chemical additives on the pulp fibers. The unretained chemical additive or additives, being water soluble or dispersible, are free to attach itself to other pulp fibers within the tissue sheet as the water is drained from the tissue sheet. The unretained chemical additive may also be removed with the process water during dewatering. As the process water is recycled in the tissue making process, the concentration of the chemical additives may build up in the system and again are free to attach itself to other pulp fibers within the tissue sheet.
Hence, in the case of both multi-layered and blended tissue sheets, despite the treatment of individual pulp fiber species, chemical contamination by chemical additives from treatments of other pulp fiber species may occur. Thus, despite attempts to keep the chemical additives from contaminating other pulp fibers, such as debonder agents, using the example from above, becoming attached to softwood kraft pulp fibers and strength agents becoming attached to hardwood kraft pulp fibers may occur, resulting in an overall detriment to tissue product quality and low chemical additive performance. At other times, certain chemical additives may not be compatible with other chemical additives being used in the tissue making process. Such incompatible interactions may be detrimental to the efficiency of the tissue making process, causing issues such as felt and fabric filling, deposit formation either in the tissue sheet or on process equipment, or effect the downstream efficiency of such things as creping adhesives.
U.S. Pat. No. 6,423,183, issued on Jul. 23, 2002 to Goulet et al. discloses a process to reduce levels of unadsorbed chemical additives in the tissue making process water by treating a pulp fiber slurry with an adsorbable, water soluble or water dispersible chemical additive, dewatering the pulp fiber slurry to a consistency of about 20 to about 30 percent to remove the unretained adsorbable chemical additive, redispersing the dewatered pulp fiber slurry at a consistency of about 3 to about 5 percent, further diluting the pulp fiber slurry, forwarding to a stratified headbox and forming a layered tissue product using conventional tissue making processes. Process water contamination is reduced by insuring that the filtrate containing the unretained chemical additive is not brought forward in the tissue making process. The effects of unretained chemical additives are reduced, but unretained chemical additives may be still present in the process water that is forwarded with the dewatered pulp fiber slurry.
Many methods require that a chemical additive be substantive to the pulp fibers as the chemical additive is applied to the pulp fibers while the pulp fibers are in a dilute slurry with water. As such, one skilled in the art would not be expect the tissue making process of the present invention to work with hydrophobic, low water solubility chemical additives such as polysiloxanes, mineral oils, and the like. While such hydrophobic, low water solubility chemical additives may be made into water dispersible emulsions using surfactants, generally these chemical additives may have poor adsorption onto pulp fibers and unless the resulting emulsion is evaporated to dryness to separate the emulsified hydrophobic chemical additive from the emulsifying particle, the emulsified hydrophobic chemical additive may be easily stripped from the pulp fibers when the pulp fibers are reslurried in the tissue making process. Even if the process disclosed in U.S. Pat. No. 6,423,183, discussed above, chemical additive systems employing poorly substantive chemical additives may show cross contamination of the chemical additives across the various pulp fiber species in the tissue sheet as well as unacceptably poor retention of the chemical additives.
The topical or surface softness of a tissue sheet, and ultimately the resulting tissue product, may be achieved by topically applying a softener agent to the surface of the tissue sheet and/or tissue product. Typically, topical softener agents are generally non-ionic and hydrophobic. One effective softener agent may be polysiloxane. Polysiloxane treated tissue sheets are described in U.S. Pat. No. 4,950,545, issued on Aug. 21, 1990 to Walter et al.; U.S. Pat. No. 5,227,242, issued on Jul. 13, 1993 to Walter et al.; U.S. Pat. No. 5,558,873, issued on Sep. 24, 1996 to Funk et al.; U.S. Pat. No. 6,054,020, issued on Apr. 25, 2000 to Goulet et al.; U.S. Pat. No. 6,231,719, issued on May 15, 2001 to Garvey et al.; and, U.S. Pat. No. 6,432,270, issued on Aug. 13, 2002 to Liu et al., which are incorporated by reference to the extent that they are non-contradictory herewith. A variety of substituted and non-substituted polysiloxanes may be used.
While polysiloxanes may provide improved softness in a tissue sheet, there may be some drawbacks to their use. First, polysiloxanes may be relatively expensive. Only polysiloxane on the outermost surface of the tissue sheet may contribute to topical or surface softness of the tissue sheet. Polysiloxanes may be effective debonding agents. However, when present in the z-direction of the tissue sheet, the polysiloxanes may negatively impact the strength of the tissue sheet while contributing to the bulk softness of the tissue sheet from debonding. Polysiloxanes and other hydrophobic chemistries tend to be poorly retained in the wet end of the tissue making process, and therefore may require topical application to a formed tissue sheet. This topical application usually involves applying the chemical additive as an emulsion to the tissue sheet using spray or printing applications. As tissue sheets are relatively thin and non-dense, topical printing and spraying may cause significant penetration of the chemical additive in the z-direction, and hence, contamination of the various pulp fiber species with the topically applied chemical additive even in a layered tissue sheet.
Therefore, there is an interest for preparing tissue products containing hydrophobic chemical additives, such as polysiloxane, wherein the hydrophobic chemical additive is selectively applied to only certain pulp fibers within the tissue sheet. There is an interest for the incorporation of hydrophobic chemical additives in the wet end of the tissue making process, avoiding the need for additional application equipment after the tissue machine and whereby the hydrophobic chemical additive is substantially located on specific pulp fiber species. There is an interest in minimizing cross contamination of pulp fibers not treated with the hydrophobic chemical additives so as to improve the performance of the hydrophobic chemical additive in the tissue sheet. For example, if polysiloxane is used, minimizing the z-directional penetration of the polysiloxane within the tissue sheet may provide more polysiloxane on the surface of the tissue sheet and better topical or surface softness of the tissue sheet is achieved at lower levels of polysiloxane. By avoiding cross contamination of strength layers within the tissue sheet, the polysiloxane does not contribute to significant strength loss within the tissue sheet, providing softer tissue sheets, and ultimately, tissue products comprising higher strength levels.